In the manufacture of paper and certain textile products and in other applications such as food drying, it is common practice to apply heat to a continuous moving web of paper or web of a textile product or other material to dry the web in the manufacturing process. Because of the enormous quantities of textiles, papers, and other products which are continuously manufactured, an enormous amount of heat must be generated in the various drying processes. Both direct and indirect drying processes have been used, but the indirect drying systems are used more frequently for materials with relatively high water content.
Where it is possible, it is desirable to use the heat generated in a dryer system as effectively as practical so as to reduce the expense of generating the heat for the system. In many situations, it is possible to recover and recirculate the heat generated in such a system, and the recovery and reuse of the heat usually results in substantial savings to the user.
For example, a typical paper dryer used in a paper manufacturing process might include a large substantially closed housing which confines eighty rotary drums or cylinders about which the web of paper travels. The dryer housing may be three hundred feet long, fifty feet high and thirty feet wide, and the continuous web of paper is fed into one end of the housing and about the rotary cylinders in the housing and then out of the other end of the housing. Heat in the form of steam is injected into the rotary cylinders so as to heat the web passing about the cylinders, and hot dry air is passed through the housing to heat the web of paper and to remove the moisture from the paper. It is estimated that paper dryers of this type use approximately 40% of the energy required to operate a paper manufacturing plant, and that paper manufacturing plants use as much as 7% of the energy in the United States. Because of this enormous energy requirement to manufacture paper, it is highly desirable to recover and recirculate the heat generated for the purpose of operating paper dryers.
While energy recovery devices have been conceived for paper and other dryers in the past, as exemplified by U.S. Pat. Nos. 3,798,785 and 2,492,754, the amount of energy recovered or method of operation of the prior art devices is not satisfactory.